Description of experiment
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Needed compounds:
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chrome alum : KCr(SO4)2 . 12H2O
hydrochloric acid : HCl
sodium hydroxide : NaOH
benzotriazole : C6H4 N3H
Class:
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elem=C,Cr
coordination
redox
Summary:
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Benzotriazole is not oxidized by air. Apparently it forms a coordination
complex with chromium (III).
Description:
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Add some solid benzotriazole to water: The solid does hardly/not dissolve.
Add some solid NaOH: The NaOH dissolves and the benzotriazole now also quickly
dissolves. The resulting liquid becomes clear and colorless.
Add an excess amount of HCl (10% by weight): The liquid remains clear and
colorless.
Add a small quantity of a solution of chrome alum: Liquid turns turbid for
a moment, but after this initial transient it becomes green and clear.
Add more HCl (10% by weight): Liquid turns greyish green and clear.